This invention relates generally to the field of wireless communications systems. More specifically, it relates to cellular communications system base stations that use adaptive antenna arrays to increase system performance.
Wireless communication system base stations conventionally have multiple antennas for communicating with mobile transceivers. Normally, however, only one antenna is selected at one time for communicating with a given mobile. More recently, researchers have been investigating the use of adaptive (smart) antenna arrays at base stations. Using sophisticated signal processing, multiple antennas of the array can be used simultaneously for communicating with a given mobile. Such smart antenna arrays, however, generally require more antenna elements than normally exist on conventional antenna towers. Moreover, in order to provide sufficient channel estimation, beam forming, and spatial diversity, conventional adaptive antenna array techniques require that the physical dimensions of the antenna array be significantly larger than the antenna platforms of existing antenna towers. Unfortunately, increasing the size of the antenna array increases the cost in setting up and maintaining the antenna array.
In one aspect of the present invention, a wireless communications base station is provided having a compact dual-polarized antenna array. The dual polarized antenna array is characterized by a small antenna cross-section, thereby making it very suitable for base stations located in urban areas where zoning is a major issue. Although the dual polarized antenna array is compact, it nonetheless provides sufficient diversity, especially in an urban environment where signal polarization is received in a highly random manner. Compared with spatial diversity, polarization diversity allows the use of a much smaller antenna array. The present invention provides techniques for implementing both polarization diversity and direction finding in a single antenna array.
According to the present invention, each antenna element of the array is dual-polarized, i.e., each antenna element has two orthogonal polarizations that can be independently selected. These dual-polarized antenna elements are adaptively controlled during transmission and reception to provide increased performance using a compact antenna array.
For signal reception, the antenna elements of the antenna array are divided into two groups, a first group and a second group. All antenna elements in the first group have a first polarization, and all antenna elements in the second group have a second polarization orthogonal to the first polarization. In other words, each antenna in the first group has a polarization orthogonal to each antenna in the second group. Since the first and second groups of antenna elements have orthogonal polarizations, reception polarization diversity can be advantageously used for direction finding and fading mitigation (fading diversity) in a single small antenna array. For direction finding, each group of antenna elements can be used independently, resulting in a first direction from the first group and a second direction from the second group. The first and second directions can then be combined to estimate the direction of arrival of received signals. For fading mitigation, signals can be received separately at the first and second antenna groups, then adaptively combined to mitigate the effects of fading.
For signal transmission, all the antenna elements in the antenna array are grouped together and set to have the same polarization. Because signals are then transmitted with the same polarization, the antenna gain for signal transmission is maximized. The common polarization can be set to either one of the two orthogonal polarizations. Alternatively, both polarizations can be simultaneously selected in each antenna element to produce a circular polarization for the signals transmitted from the antenna elements.
In addition to the above techniques, the polarizations of the antenna elements in the array can be selected adaptively in transmission or reception according to the specific signal polarization characteristics of each mobile. For example, in reception, the combination of antenna polarizations can be selected that maximizes the signal-to-interference ratio (SIR) for a given mobile. These polarization settings can then be used for transmission as well to improve reception at the mobile.